The present invention relates to electromechanical filter devices and, more particularly, to reed type electromechanical filter devices for use with various radio communication equipment and electrical control devices.
A reed type electromechanical filter device uses a generally fork-shaped vibrator element having a pair of spaced parallel arm portions which are adapted to vibrate in lateral directions about a nodal axis located in an intermediate base portion through which the arm portions merge into each other. Piezoelectric ceramic plates are fixedly mounted on the outer side faces of the arm portions, respectively, of the vibrator element by means of adhesive. The piezoelectric ceramic plates operate as electromechanical transducer elements converting mechanical vibrations of the arm portions of the vibrator element into currents with frequencies within an audible range. The filter devices of this character have been extensively used in the field of communication and control devices and instruments by reason of the stable frequency characteristics.
Because, however, of the fact that the arm portions of the vibrator element are usually allowed to warp at relatively large angles between side walls of a housing which encloses the vibrator element, problems have been encountered which result in critical deterioration of the performance quality of the filter device. When, for example, the filter device happens to fall to a hard floor surface or struck by a hard object and undergoes a violent impact, the arm portions of the vibrator element tend to be unusually warped and would forcefully strike against each other or the inner faces of the side walls of the housing. If the arm portions of the vibrator element are thus seriously defomred or subjected to a violent impact, the piezoelectric ceramic plates susceptible to mechanical shocks are liable to be ruptured and/or cracks may be produced in the layers of the metal-to-metal adhesive bonding the ceramic plates to the arm portions of the vibrator element. This causes diminution of the resonance frequency of the filter device and gives rise to an increase in the insertion loss of the device so that the filter device becomes no longer serviceable. Such a damage will be particularly serious when an impact is applied to the filter device in a direction in which the arm portions of the vibrator element are to vibrate.
As is well known in the art, moreover, it is a widely accepted practice to have the resonance frequency of the filter device minutely adjusted during production of the filter device by applying controlled quantities of adhesive to the arm portions of the vibrator element for furnishing the vibrator element with additional masses. The adhesive is, however, initially in a liquid state and for this reason tends to creep or flow on the surfaces of the vibrator element so that extreme difficulties are encountered in accurately controlling the quantities, locations and distributions of the additional masses on the vibrator element. If the additional masses of the adhesive applied to the surfaces of the vibrator element fail to have proper quantities and are distributed irregularly, the equilibrium between the modes of vibration of the two arm portions of the vibrator element will be destroyed and, again, the filter device will become useless.
The present invention contemplates elimination of all these drawbacks which have been encountered in the prior art reed type electromechanical filter devices of the described nature.